I live in uk and I want to visit china for a week, what documents will I need.

Steven, you can apply for a tourist L type and ask your girlfriend issue an invitation letter to you; inside the letter, she should state that you will stay with her after your entry. In this case, you don’t need to provide details of accommodation. And you just need to provide those documents: the application form and a passport-sized color photo, your passport and copy of its data page, and the invitation letter from your girlfriend.

do i need any supporting documents like payslips, bank statements etc?

I don’t think you need to provide proof of your financial support.